Conventional cation and proton conducting membranes typically comprise a sheet of a homogeneous polymer, a laminated sheet of similar polymers, or a blend of polymers. A variety of polymers have been demonstrated to be cation conductors and some of the membranes produced using these polymers are highlighted in Table 1. All of these membranes, with the exception of the Gore Select.TM. membrane, arc homogeneous polymers. The Gore Select.TM. membrane is a polymer blend.
TABLE I ______________________________________ Polymers Used as Ion Conductors Source Name Polymer Structure ______________________________________ DuPont Nafion .RTM. Perfluoro side chains on a PTFE backbone Dow Perfluoro side chains on a PTFE backbone W. L. Gore Gore Select .TM. Perfluoro side chains on a PTFE backbone in a matrix Ballard Trifluorostyrene backbone, with derivatized side chains Maxdem Poly-X .TM. Polyparaphenylene backbone DAIS Corp. Sulfonated side chains on a styrene- butadiene backbone Assorted Sulfonated side chains grafted to PTFE and other backbones ______________________________________
Two of these materials, the membranes from DuPont and Dow, have relatively similar compositions and structures. These structures are illustrated in FIG. 1. Both of the polymers are perfluorosulfonic acids (PFSA's), which are solid organic super-acids, and both membranes are produced as homogeneous sheets. The active ionomer component of the Gore blend is also a PFSA material.
All of those polymer materials rely on sulfonate functionalities (R--SO.sub.3 --) as the stationary counter charge for the mobile cations (H.sup.+, Li.sup.+, Na.sup.+, etc.), which are generally monovalent. The most commonly proposed mechanism for this conduction, through essentially solvated cations, is illustrated in FIG. 2, which is a schematic drawing of the commonly proposed structure for perfluorosulfonic acid (PFSA) polymers, as typified by NAFION (a registered trademark of Dupont of Wilmington Delaware). One difficulty associated with this approach to cation conductivity is that the polymer membrane requires the presence of water for conductivity. As shown in FIG. 3 increasing water content increases conductivity at all temperatures. This dependence on water is the weak point of membranes that rely on sulfonic acid groups for their conductivity. As long as proton exchange membranes (PEM) membranes are kept hydrated, they function well, but when they dry out, resistance rises sharply.
The need for a PEM source of moisture besides the water generated at the cathode to maintain the amount of water in the membrane to maintain conductivity in PEM fuel cell membranes has been recognized for as long as PEM fuel cells have been known. A wide variety of methods have been developed to keep membranes supplied with water. These methods typically require adding water as either vapor or liquid to the gas streams entering the cell or adding water directly to the membrane.
There are a number of reasons that water is so easily lost from PEMs, even as it is being generated at the cathode. The vapor pressure of water over a saturated PEM is nearly as great as it is over pure water. This means that at a temperature of 100.degree. C., a full atmosphere of water vapor is required to keep the membrane saturated.
The water carrying power of gaseous oxidizer streams are quite substantial. It is difficult to operate a fuel cell with an air flow of less than twice the amount required to supply a stoichiometric amount of air for oxidation of the fuel (commonly termed two-fold stoichiometry). If a fuel cell is operated at ambient pressure, operating at a temperature of 55.degree. C. will result in the exiting air stream carrying all of the water produced by the cell at two-fold stoichiometry. Operating at temperatures above 55.degree. C. with the same air flow will cause a PEM membrane to become progressively drier. Increasing the operating pressure of the cell or stack will permit operation at higher temperatures, but the price of higher pressure is increased parasitic power losses.
If a proton-conducting membrane could be developed with improved water retention or a reduced dependence on free moisture for proton conduction it would be possible to operate a proton conducting membrane fuel cell with less water, with no water, or at higher temperatures. This would provide simpler, lighter fuel cell stack designs.
There is a related problem that only applies to direct methanol fuel cells (DMFC's) which is referred to as "methanol crossover." Typical PFSA fuel cell membranes have a higher affinity for methanol than they do for water, as is clearly illustrated in FIG. 4. In a DMFC, the crossover process relates to the permeation of absorbed methanol through the membrane from anode to cathode. In general, it has been found that rate of methanol crossover through a PEM is proportional to the methanol concentration in the fuel feed stream. Therefore, a proton conducting membrane that requires less water to maintain its conductivity will also exhibit a reduced methanol flux.
Methanol crossover substantially impedes the performance of direct methanol fuel cells. First, methanol that crosses over represents lost fuel value and, therefore, a lower fuel efficiency. Furthermore, when that methanol arrives on the other side of the PEM, it is oxidized by the cathodic electrocatalyst which depolarizes the electrode. Oxidation of methanol at the cathode increases the amount of air, or oxygen, that the cell or stack requires, since a molecule of methanol oxidizing on the cathode requires the same 11/2 molecules of oxygen (O.sub.2) as one being consumed at the anode. Since none of the energy from this oxidation is being extracted as electricity, it all ends up as waste heat, increasing the cooling load on the cell. A proton conducting membrane with substantially reduced methanol crossover would represent a significant improvement in DMFC's.
Alternatives to polymer proton conductors include oxide proton conductors. A wide variety of metal oxides are proton conductors, generally in their hydrated or hydrous forms. These oxides include hydrated precious metal containing oxides, such as RuOx (H.sub.2 O).sub.n and (Ru--Ti)O.sub.x (H.sub.2 O), acid oxides of the heavy post transition elements, such as acidic antimony oxides and tin oxides, and the oxides of the heavier early transition metals, such as Mo, W, and Zr. Many of these materials are also useful as mixed oxides. Some oxides which do not fit this description may be useful as well, such as silica (SiO.sub.2) and alumina (Al.sub.2 O.sub.3), although these are generally used as, or with, modifiers.
The number of metal oxides with the potential to serve as proton conductors is too large to fully discuss in detail here. This group, which can be summarized as those elements forming insoluble hydrated oxides that are not basic, includes not only known proton conductors, but oxide superacids that will furnish a multitude of free protons in the presence of an aqueous medium. These are shown in bold in FIG. 5. Many other elements which are not included in this list may be useful in conjunction with these elements as modifiers. An example of this is the inclusion of phosphorus in the structure of Keggin ions which consist primarily of a tungsten or molybdenum oxide framework. While the compounds encompassed in the description above have some degree of proton mobility, not all of those oxides have adequate proton mobility to be useful as components in composite membranes. Some particularly useful examples are discussed below.
Zirconium phosphate, specifically .alpha.-zirconium phosphate, whose structure is shown in FIG. 6, is known to be an excellent proton conductor when tested as a powder at ambient temperature. Under these conditions the compound is hydrated (Zr(HPO.sub.4).sub.2 (H.sub.2 O), and most of the conductivity is the result of protons migrating over the surface of the individual crystallites. Above 120.degree. C. the water of hydration is lost and the conductivity drops substantially to a value representing the bulk conductivity of the solid, which increases from 1.42 .mu.S at 200.degree. C. to 2.85 .mu.S at 300.degree. C. With this combination of properties, .alpha.-zirconium phosphate is suitable for use in either low temperature (&lt;100.degree. C.) fuel cells, or in higher temperature (&gt;150.degree. C.) fuel cells.
This structure is not unique to .alpha.-zirconium phosphate. Hafnium, titanium, lead and tin all have phosphates that crystallize in this structure. These compounds have substantially less free volume in their structures than the zirconium compound, and are expected to show lower proton mobilities.
Tungsten and molybdenum offer two groups of proton conductors. The first of these groups are the simple, fully oxidized metals, as exemplified by tungsten trioxide (WO.sub.3). This compound has been the subject of much interest due to its electrochromic properties. This oxide can be repeatedly electrochemically reduced in the solid state, with a color shift from light yellow to blue, and reoxidized back to the light yellow form. This property has been used to produce electrochromic windows that can be lightened and darkened as desired. This reaction occurs without any significant rearrangement of the crystal lattice. As a result, maintaining charge neutrality requires a cation (proton) to diffuse into the structure and reside on an interstitial site. By maintaining an appropriate bias across an oxide film, a proton flux can be maintained.
The second family of tungsten and molybdenum compounds demonstrated to have high protonic conductivity are the hetero.sup.- and homo.sup.- polymolybdates and polytungstates. This description encompasses a broad range of compounds with widely varying compositions, all of which are based on the fusion of groups of MO.sub.6 (M=Mo, W) octahedra by edge or corner sharing. These ions (and they are all anions) have a generic formula of (X.sup.k+ M.sub.n O.sub.(3n+m)).sup.(2m-k)- where k is the positive charge of the heteroatom, if any, and m is the number of unshared octahedral comers in the structure. An example of a typical structure is illustrated in FIG. 7. The large cage in the center of the ion can host a heteroatom, such as P or As, which lowers the net charge on the ion. The exact structure formed is a function of temperature and pH, with interconversion between frameworks occurring with changing conditions.
The variety of compounds in this category continues to expand, with new compounds being synthesized and characterized regularly. Some of them, such as the (Mo.sub.16 V.sub.14 O.sub.84).sup.14- ion, have very complex structures.
Compounds in this family have been demonstrated to have room temperature proton conductivities as high as 0.17 S cm.sup.-1 for H.sub.3 W.sub.12 PO.sub.40 *29 H.sub.2 O and 0.18 S cm.sup.-1 for H.sub.3 Mo.sub.12 PO.sub.40 *29 H.sub.2 O (this is over an order of magnitude greater than the conductivity of Nafion.RTM. measured under the same conditions). These compounds have the thermal stability to remain proton conducting above 200 C, albeit with a reduced conductivity. Not only are these compounds proton conductors in their own right, but when silica gel is doped with H.sub.3 W.sub.12 PO.sub.40 *29 H.sub.2 O while it is being formed from tetraethoxysilane (TEOS) by a sol-gel reaction, then the product is an amorphous proton conductor with a conductivity that varies with the concentration of the tungstate, which may be present at up to about 50 percent by weight.
In one series of experiments, solutions of some of these acids were immobilized in polymer sheet matrices and the resulting electrolyte membrane used in a fuel cell operated at room temperature which showed good performance. However, the significant weakness of this approach is that the electrolyte is present as a liquid and, therefore, is subject to displacement out of the matrix if a pressure imbalance occurs.
In another series of experiments the acids were used in solid form, as either the pure acids, or in combination with a salt of the acid. In some cases a small amount (typically 0.5%) of a resin described as ethylene tetrafluoride powder was added to the acid. The addition of the resin improved the physical properties of the finished electrolyte, but even with this addition, membranes thinner than 2 mm could not be produced. The lower resistivity of the solid acid was not sufficient to overcome the resistive losses produced by a membrane nearly 16 times as thick as a typical polymer membrane (2 mm vs. 0.127 mm), and the resulting fuel cell exhibited poor performance. Tungsten oxides have also been used as electrocatalyst supports, and in this role have demonstrated an ability to enhance oxygen reduction for the platinum catalyst on the support.
Another family of compounds that have been demonstrated to have high proton conductivity are the oxoacids of antimony. These compounds have a structure consisting of edge or corner shared SbO.sub.6 octahedra, as shown in FIG. 8. Unshared oxygens are protonated (i.e., hydroxyls) and charge neutrality is maintained by exchangeable external cations. In these acids, antimony can be in either the +3 or +5 oxidation states, or a mixture of the two, depending on the synthesis conditions and subsequent treatment. The key step in the synthesis is the hydrolysis of SbCl.sub.5, with or without hydrogen peroxide, generally carried out at 0.degree. C. The more oxidizing the hydrolysis conditions, the larger will be the fraction of the antimony in the +5 oxidation state in the final product, and with a sufficiently oxidizing hydrolysis solution it is possible to obtain acids with all of the antimony in the +5 state. The acid precipitates as an insoluble white powder having a pyrochlore-type framework structure (based on cubic symmetry). The powder is thoroughly washed and dried at room temperature before further use.
Antimonic acids are dehydrated on heating in dry air, with most of the water lost at around 140.degree. C. As long as the material is not heated above 200.degree. C. it will reabsorb water from air, even under normal room conditions, and return to its original weight. Heating to temperatures above 300.degree. C. lead to deoxygenation, with the Sb.sup.+5 present reverting to Sb.sup.+3.
Thin films of antimonic acid have been produced on conductive surfaces by electrophoretically depositing fine particles suspended in a solution of ammonium hydroxide in acetone. Although the resulting layers were shown by SEM to be smooth, no information was given on whether or not they were pore free, a requirement for this application.
Like tungsten and molybdenum, tantalum and niobium form highly charged complex polyanions, as illustrated in FIG. 9. These materials are also facile cation exchangers capable of proton conduction and subject to irreversible dehydration if heated above 100.degree. C.
These families of inorganic ion exchangers have significant differences, but they also have three common features that make them candidates for use as proton conducting electrolytes in fuel cells. First, they all have easily exchangeable protons. Second, they all have open framework structures with channels to provide low resistance paths for the mobile protons to move along. Third, they all retain their proton conductivity at temperatures in excess of 200.degree. C., and in most cases, in excess of 300.degree. C. This last characteristic would appear to make it possible to use these compounds in fuel cells operating at slightly elevated temperatures, as well as at the same low temperatures (&lt;100.degree. C.) where conventional PEM fuel cells are used. Unfortunately, all of these oxide proton conductors are ceramic materials which are difficult to fabricate into thin, pin hole free, films.
There are other inorganic compounds, with significantly different structures, which also offer a high degree of proton mobility. These inorganic compounds include solid superacids and oxides with highly hydrated surfaces as shown simplistically in FIG. 10. In both cases, the proton conductivity comes from protons diffusing over the surface of individual crystallites, or particles in the case of amorphous materials. This effect has already been described for fully hydrated .alpha.-zirconium phosphate. The Grothaus proton hopping occurring here is the same process that is presumed to account for the proton conductivity of PFSA membranes, polyphosphoric acid, and tungstic acid, as illustrated in FIG. 11.
Hydrated ruthenium oxides are one of the materials known to be capable of supporting a significant ionic current through the surface proton hopping mechanism described above. However, pure RuOx (H.sub.2 O).sub.n would not be acceptable for use in electrolyte membranes since this compound is a metallic conductor. As such, it would electrically short circuit any cell in which it is used.
Ruthenium oxide "stuffed" Nafion.RTM. membranes have been tested as electrolyte membranes in direct methanol fuel cells and were demonstrated to reduce methanol crossover. Unfortunately, in this incarnation they were also found to reduce proton conductivity significantly.
A recently reported aerogel synthesis has been demonstrated to be particularly effective in generating proton conducting materials, largely because the products of this reaction have very high surface areas with a high degree of hydroxyl terminations and good electrical separation of local RuO.sub.x domains. (Ru.sub.0.32 Ti.sub.0.68)O.sub.2 is a mixed conductor with both electrons and protons acting as charge carriers, and flowing in opposite directions. When normally synthesized as a bulk material, the majority of the current is carried by electrons. When the material is synthesized as an aerogel, with a greatly increased surface area, the majority of the charge is carried by protons. This is a clear demonstration of the surface protonic conductivity of RuO and a clear route to a way of utilizing it. The key to the aerogel process is keeping the widely dispersed sol-gel network, which is produced by the hydrolysis of a relatively dilute solution of metal alkoxides, separated as the solvent is removed. A similar effect can be harnessed in the production of membranes, as described in a later section of this disclosure.
Sulfated zirconia is an amorphous solid super acid that has recently received significant attention as an acid catalyst primarily for use in hydrocarbon conversions and as an acid support for other catalysts. Titanium oxides, and titanium-aluminum oxides, have been shown to have similar properties, but this discussion will focus on the better known zirconia compounds.
These materials are generally viewed as amorphous metal oxides with sulfate groups attached to their surface. They are produced by a variety of routes. The classical method is precipitation of amorphous Zr(OH).sub.4 by treating an aqueous solution of a zirconium salt with a base followed by sulfonation of the gel with either sulfuric acid or ammonium sulfate. The amorphous Zr(OH).sub.4 can also be produced by a sol-gel method, and sulfated in the same way. Both of these methods are essentially two-step syntheses. Higher surface area materials can be produced by the direct reaction of sulfuric acid with the alkoxide precursor. The catalyst is activated before use by calcination at temperatures between 400.degree. and 650.degree. C. Although these materials are strong Bronsted acids, like PFSA materials, they require water for the formation of free protons.
Solids with similar properties can also be produced with alumina (Al.sub.2 O.sub.3) serving in place of zirconia. These materials are produced by combining a salt, such as Li.sub.2 SO.sub.4 or RbNO.sub.3, with the corresponding aluminum salt and sintering the mixture to convert the aluminum salt to an alumina matrix. The guest salt remains relatively unchanged. These materials can be pressed to form tablets about 1-2 mm thick, which were tested as fuel cell electrolytes. When operated at 400.degree. C. they were found to produce promising results, with single cell potentials as high as 0.75 V observed at current densities of 200 mA/cm.sup.2. The conductivity was attributed to protons moving along sites formed by the salt in the alumina matrix based on IR evidence of H--SO.sub.4 coordination in the lithium containing electrolyte. However, because of the high temperature required for conductivity, these materials are not considered promising for use in a polymer bonded system.
All of the oxides described above are potentially useful as proton conductors, if they could be fabricated into sufficiently thin sheets that the conductivity would be similar to conventional polymeric membranes. The inability to produce thin sheets is a key weakness of materials produced by the approach or method used by Nakamora et al. (U.S. Pat. No. 4,024,036.)
In addition to inorganic cation conductors, inorganic-organic composite membranes are potentially useful for electrochemical applications. PFSA membranes, such as Nafion.RTM., have been filled with 12-phosphotungstic acid (H.sub.3 W.sub.12 PO.sub.40), an inorganic proton conductor. These membranes have been demonstrated to have better water retention and, consequently, better conductivity at temperatures above 100.degree. C. than the same membranes in their unfilled form. The goal was to develop membranes for PEM fuel cells that could be operated at elevated temperatures to ameliorate the problem of CO poisoning for anode electrocatalysts. The addition of 12-phosphotungstic acid to the polymer electrolyte permitted operation at temperatures up to 120.degree. C., but no evidence was shown for improved CO tolerance.
In U.S. Pat. No. 5,523,181, Stonehart et al. describe a composite membrane useful for PEM fuel cells consisting of high surface area silica, preferably in the form of fibers, as a filler with a variety of polymers capable of exchanging cations with solutions as the matrix. These membranes are produced by suspending the inorganic phase in a solvent appropriate for the dissolution of the polymer and blending the suspension with a solution of the polymer in the same solvent. Membranes are formed by evaporating the solvent in a controlled manner to produce a thin film of the composite. The silica is selected to maximize its affinity for water and ability to retain water. They demonstrate reduced electrical resistance in fuel cells operating under conditions of low humidification. The improved performance is attributed to improved water retention by the silica, and improved back diffusion of water from the cathode to the anode along the silica fibers with the back diffusing water replacing water removed by electroosmotic transport. They have not attributed any contribution to the overall proton conductivity to the silica.
In U.S. Pat. No. 5,512,263, McIntyre describes a composite membrane produced using an ionically conductive polymer together with an electrically conductive filler phase. This membrane permits the construction of an internally shorted fuel cell, which is described as useful for the synthesis of hydrogen peroxide. Since all of the electrical current flows internally within the membrane, there is no external electrical control or monitoring of the reaction. This lack of control may contribute to the relatively low efficiency of their process.
In U.S. Pat. No. 5,682,261, Takada et al. disclose a three phase system for producing a composite membrane. A Br.o slashed.nsted acid, typically a strong mineral acid is adsorbed onto the surface of finely divided silica and this mixture is combined with a thermoplastic binder to produce a proton conducting membrane. In this membrane the primary conductivity is due to free protons in the acid. This membrane has been found to be useful as an ion conductor for electrochromic windows and for fuel cells.
In U.S. Pat. No. 5,334,292, Rajeshwar et al. describe a composite consisting of an electron conducting polymer (as opposed to an ion conducting electrolyte) and catalytically active metal particles. The polymers they use are polypyrrole and polyanaline which are polymerized electrochemically on a conductive surface. This composite is described as being useful as a supported electrocatalyst where it is desirable to suspend precious (e.g., Pt, Pd, Ag, Ru, etc.) electrocatalytically active particles in an inexpensive conductive matrix to minimize the amount of precious metal used.
Inorganic-organic composite membranes may also be useful for a variety of other applications. These composites may include a Nafion.RTM. matrix and a semiconductor filler, where the semiconductors generally selected are those known to show activity for carrying out photocatalytic reactions, such as CdS, CdSe, FeS.sub.2, ZnS, TiO.sub.2, and Fe.sub.2 O.sub.3. The composites produced arc useful for carrying out reactions such as the photocatalytic decomposition and oxidation of organic compounds and even the fixation of nitrogen.
In their article entitled "Nafion/ORMOSIL Hybrids via in Situ Sol-Gel Reactions. 3. Pyrene Fluorescence Probe Investigations of Nanoscale Environment," (Chemistry of Materials, 9, 36-44, (1997), Mauritz et al. describe PFSA-silica composites by the hydrolysis of tetraethoxysilane (TEOS) inside the polymer matrix. The inorganic-organic ratio can be varied over a wide range, as can the properties of the inorganic phase, permitting the properties of the final composite to be tailored for specific applications. These composite materials have been demonstrated to have improved selectivity for gas separation when compared to the unfilled polymer. Mauritz et al. have also demonstrated the ability to produce nanophase composites with TiO.sub.2, titaniasilicate, and aluminasilicate inorganic phases.
A number of authors have described PFSA membranes filled with transition metal complexes. Compounds used in this way have included complexes of ruthenium, rhodium, palladium, silver, rhenium, iron, and manganese. In these composites, the polymer serves to immobilize and stabilize the ionic or molecular species, which continues to exist in essentially the same form as it does in solution. These complexes usually serve as catalysts, with the transition metal species serving the same function as it does when used as a homogeneous catalyst, but with improved catalyst life and recoverability. In the case of silver, the filled membrane was found to have improved selectivity for separation of dienes from monoenes. While these materials are sometimes described as composites, they are more accurately described as immobilized homogeneous catalysts with a polymer as the support.
Therefore, there is a need for ionically conducting materials or composites exhibiting high cation conductivity and reduced dependence on water. It would be desirable if these materials or composites performed well not only at temperatures below about 100.degree. C., but also above about 150.degree. C. or more. It would also be desirable if the materials or composites were suitable for use as PEMs in fuel cells, particularly in fuel cells using reformate fuels which may contain carbon monoxide.